Top 25 SQL Interview Questions And Answers|SQL Support Interview Questions And Answers| Simplilearn

Summary

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Takeaways

• 😀 SQL (Structured Query Language) is essential for managing and manipulating relational databases, enabling operations like querying, updating, and deleting data.
• 🛠️ Understanding the order of SQL query execution is crucial: FROM, WHERE, GROUP BY, HAVING, SELECT, ORDER BY, and LIMIT.
• 🔍 SQL databases are relational with structured schemas, while NoSQL databases handle large volumes of unstructured data in real-time applications.
• 🔗 Inner join returns rows with matches in both tables, whereas left join includes all rows from the left table and matched rows from the right.
• 📊 Normalization reduces data redundancy and improves integrity, while denormalization combines tables to optimize query performance.
• ⚖️ ACID properties (Atomicity, Consistency, Isolation, Durability) ensure reliable database transaction processing.
• 📑 GROUP BY organizes data for aggregation, while HAVING filters results based on conditions applied to grouped data.
• 📈 Indexing enhances data retrieval speed by creating data structures on one or more table columns.
• 🔄 A stored procedure is a precompiled collection of SQL statements that can be executed as a single unit.
• 📅 To prevent SQL injection, use parameterized queries or prepared statements to treat user input as data, not executable code.

Q & A

What is SQL and its primary purpose?

SQL, or Structured Query Language, is a programming language designed for managing and manipulating relational databases. It performs operations such as querying, updating, inserting, and deleting data.

What distinguishes SQL databases from NoSQL databases?

SQL databases are relational and use a structured schema, making them suitable for complex query-intensive transactions. In contrast, NoSQL databases are non-relational and are better suited for handling large amounts of unstructured data and real-time applications.

Explain the difference between INNER JOIN and LEFT JOIN.

INNER JOIN returns only the rows with matching values in both tables, while LEFT JOIN returns all rows from the left table and matched rows from the right table. If there are no matches, NULL values are returned for columns from the right table.

What is normalization, and why is it important?

Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It is important as it ensures that the database is efficient and minimizes the potential for data anomalies.

What are the ACID properties in the context of database transactions?

ACID stands for Atomicity, Consistency, Isolation, and Durability. These properties ensure that database transactions are processed reliably: Atomicity ensures transactions are treated as a single unit, Consistency ensures valid state transitions, Isolation ensures concurrent transactions do not interfere with each other, and Durability guarantees that once committed, transactions remain so despite system failures.

How does indexing improve database performance?

Indexing creates a data structure that improves the speed of data retrieval operations on a database table. It allows the database engine to quickly locate and access rows that satisfy a query.

What is a stored procedure?

A stored procedure is a precompiled collection of one or more SQL statements that can be executed as a single unit. It is stored in the database and can be called by other programs or procedures.

How can you prevent SQL injection attacks?

SQL injection can be prevented by using parameterized queries or prepared statements, which ensure that user input is treated as data and not executable code.

What is the purpose of the CASE statement in SQL?

The CASE statement allows you to perform conditional logic in SQL queries, enabling different actions based on different conditions.

How do you update data in a SQL table?

To update data, you use the UPDATE statement. For example, 'UPDATE employees SET salary = 50000 WHERE department_id = some_id;' modifies the salary for employees in a specified department.